The present disclosure relates to stereoselective Ziegler-Natta catalyst compositions and polymerization reactions utilizing the catalyst composition.
Ziegler-Natta propylene polymerization catalyst compositions are known in the art. Typically, these compositions include transition metal moieties, such as titanium, magnesium and halide moieties in combination with an internal electron donor (referred to as a procatalyst); a co-catalyst, usually an organoaluminum compound; and a selectivity control agent (SCA). It is further known to utilize an SCA that is a mixture of two or more components in order to modify catalyst activity and/or adjust polymer properties. Problematic is the selection of suitable SCA mixtures as an SCA component which is effective with respect to one process or product parameter is oftentimes detrimental to another process or product parameter.
For example, third generation Ziegler-Natta catalysts typically contain a benzoate as an internal donor and use a benzoic acid ester (such as ethyl p-ethoxybenzoate) as an SCA component. This provides a self-extinguishing property to the catalyst. Benzoic acid esters, however, impart an undesirable odor to the formed polymer. This odor is detrimental as it limits the use of the formed polymer. In addition, the catalyst activity and stereoselectivity for third generation Ziegler-Natta catalysts is low. Furthermore, when benzoic acid-based SCAs are used with fourth generation catalysts that contain a phthalate as internal donor, the stereoselectivity of the polymer generated is too low for practical applications.
It would be desirable to develop a Ziegler-Natta catalyst composition with improved catalyst activity and improved productivity without sacrificing operability. Also desired is a catalyst composition that does not convey undesirable properties upon the resultant polymer.